Blown fibre installations may comprise an optical fibre cable containing empty tubes, or ducts, into which optical fibre units are installed by blowing with compressed air. The installation of optical fibres by blowing along passages in a pre-installed cable is known from EP 0 108 590. The cables are often installed a long time before the optical fibre units are blown in and the optical fibre units will often be installed by someone other than the installer of the cable. As a consequence, the person installing the optical fibre unit is quite likely to be unaware of the length of the cable. A further problem is that the cable may be damaged during installation so that one, or more, of the tubes is at least partially blocked, to the extent it is not possible for an optical fibre unit to get past the blockage.
Alternatively, blown fibre installations may comprise pre-installed ducting along which optical fibre cables or optical fibre units can be installed by blowing. Methods for installing optical fibre cables into a pre-installed duct are disclosed in U.S. Pat. No. 5,645,267 and U.S. Pat. No. 6,311,953. The abovementioned problems of knowing the length of the ducting and of damage causing blockages in the ducting may apply equally to this form of blown fibre installation.
The applicant has already addressed the problem of determining the length of pre-installed ducting and of tubes in pre-installed optical fibre cables in European Patent Applications Nos 03 253191.5 and 04 253025.3.
Damage to the ducting or tubes into which the optical fibre units or optical fibre cables are to be installed by blowing gives rise to two problems:
1) the optical fibre unit/optical fibre cable can become trapped at the point of damage; and
2) there may be a reduction in airflow through the ducting or tube caused by a reduction in the cross-section thereof, which will prevent the optical fibre unit or optical fibre cable from being installed successfully.
It is known to detect internal damage to a tube in an optical fibre cable by blowing a short length, for example 150 mm, of optical fibre unit along the tube. The test piece usually has a rounded bead crimped to its front end to reduce the likelihood of its snagging against the wall of the tube. If the test piece blows through the tube successfully, it is known that the tube is in good condition for installation of an optical fibre unit. If the test piece does not emerge from the end of the tube, it is known that there is damage causing a blockage. However, this does not identify the location of the blockage. The location can be determined by installing an optical fibre unit until it reaches the blockage. The position of the blockage can then be determined by reference to the length indicator on the blowing head.
It is an object of the invention to provide methods and apparatus for determining the location of obstructions in ducting or tubing of a blown fibre installation, without the need to attempt to install the optical fibre unit or optical fibre cable.